A secondary battery is used in portable devices, electric power tools, electric vehicles and the like. A lithium ion battery out of the secondary batteries has features of showing high operation voltage, easily providing a high output and besides having high energy density, because an ionization tendency of lithium is large. Furthermore, the lithium ion battery is also expected to be applied to a large-sized power supply such as a stationary power supply and a stand-by power supply.
Here, an alternating current impedance method is known, as a method for measuring characteristics of the secondary battery such as the lithium ion battery. For instance, Japanese Patent Application Laid-Open Publication No. 2009-97878 discloses a measurement method which analyzes a Nyquist plot that has been acquired by the alternating current impedance method, by using an equivalent circuit model.
In addition, Japanese Patent Application Laid-Open Publication No. H08-43507 discloses a method of simply estimating a degraded state or the capacity of the measured battery, by specifying a frequency at which there is a high correlation between impedance and a battery capacity.
As is shown in FIG. 1, when a measurement frequency is scanned from a high-frequency side to a low-frequency side in the alternating current impedance method, a trajectory A of the impedance, which draws a semicircle clockwise, is obtained as a Nyquist plot. The trajectory A is generated by influences of a solution resistance of the battery, a charge transfer resistance and an electric double layer capacitor. The battery has two electrodes (positive electrode and negative electrode), and accordingly the trajectory A becomes a trajectory in which at least two semicircles overlap one another. When the trajectory A is analyzed using the equivalent circuit model, characteristics of each component of the electrodes, the electrolyte and the like can be grasped which constitute the battery.
When the frequency is further lowered, a trajectory B is obtained which is a straight line having a gradient of approximately 45 degrees. The trajectory B is a Warburg impedance which is generated by an influence of ion diffusion.
Here, when the measurement frequency is lowered than that of the trajectory B, a trajectory C is obtained which is perpendicular to a Z′ (real number impedance) axis. The perpendicular trajectory C is in a state of showing a capacitive component which does not depend on an internal resistance. That is, a limiting capacitance CL (limiting impedance ZL) which has a correlation with the capacity of the battery can be easily acquired from the trajectory C. The change of an active material of the battery, that is, a degradation of the battery caused by the usage count (number of charge and discharge cycles) can be diagnosed directly from a relative value between the measured limiting capacitance CL and an initial limiting capacitance (for instance, ratio between both capacitances).
However, in order to analyze the trajectory A by using the equivalent circuit model, an expensive apparatus has been needed. On the other hand, the frequency at which the trajectory C appears is as extremely low as 0.1 mHz or less, for instance, and accordingly a long time period is required in order to acquire the limiting capacitance CL. For instance, in order to measure the capacitance at the frequency of 0.1 mHz, approximately 170 minutes are required. Furthermore, in order to obtain a result with higher accuracy, the capacitance is measured while the frequency is varied around a predetermined frequency. For this reason, it has not been easy for a user to measure the degree of degradation of the battery while the battery is used.